OLED display device, which is also known as organic electroluminescence display, has the characteristics of self-luminescence and adopts a very thin organic material coating and a glass substrate. The organic material glows when the current flows through it. Moreover, the OLED display device has a large visual angle and is significant power saving. Therefore, the OLED display device has incomparable advantages over many traditional display devices, such as liquid crystal display devices.
FIG. 1 shows a schematic diagram of an equivalent circuit for a pixel unit in an OLED display device, and FIG. 2 shows a schematic diagram of an OLED display device in the prior art.
As shown in FIG. 1, in one pixel unit, a scan signal “scan” controls the on or off state of a thin film transistor M1. In the case that M1 is in the on state, a data signal “data” is transferred to the gate electrode of a thin film transistor M2 to control the current in M2. The current flows from the power supply signal PVDD to the low potential end PVEE through the thin film transistor M2, and the organic light-emitting diode “oled” emits light in response to the current and implement the display. The organic light-emitting diode “oled” is a current-driven element, in which the current flowing through the organic light-emitting diode “oled” may be calculated from the following formula:Ioled=½×μ×Cox×W/L×(PVDD−Vdata−Vth)2 
μ: the mobility of the active layer;
Cox: the capacitance of the capacitor formed by the gate electrode and the active layer;
W: the width of the channel in the organic light-emitting diode;
L: the length of the channel in the organic light-emitting diode;
Vdata: the voltage of the data signal;
Vth: the threshold voltage of the organic light-emitting diode; and
PVDD: the voltage of the power supply signal.
Once the size of the organic light-emitting diode “oled” is determined, the values of μ, Cox, W, L and Vth are also determined. As can be seen from the above formula, for a certain organic light-emitting diode “oled”, the current flowing through the organic light-emitting diode “oled” is determined by the value of the power supply signal PVDD and the data signal Vdata.
Next, reference is made to FIG. 2. In an OLED display device, multiple pixel units arranged in a matrix are disposed in a region AA, and a driver chip “Driver IC” is disposed on one side of the OLED display device and configured to provide scan signals and data signals to the multiple pixel units. A power supply driver chip “Power IC” is also provided. Generally, only one power supply driver chip is required for the OLED display device with a small panel. The driver chip “Driver IC” is disposed on the panel of the OLED display device, and the power supply driver chip “Power IC” is disposed on a flexible circuit board or a printed circuit board.
The power supply driver chip “Power IC” supplies an input voltage PVDD to the multiple pixel units through an input path P, for driving the organic light-emitting diodes to emit lights and implement the display. In the entire display region AA, input terminals of the pixel units for input the voltage PVDD are connected together. Since the OLED display device is a current-driven device and each pixel unit has a resistance, a voltage drop is generated across the pixel units from the point AA1 nearest to the power supply driver chip “Power IC” to the point AA2 farthest to the power supply driver chip “Power IC” due to the driving current. That is, the voltage at the nearest point AA1 is larger than that at the farthest point AA2.
From the nearest point AA1 to the farthest point AA2, the currents flowing through the pixel units decrease gradually from one pixel unit to another as the voltages decrease sequentially from one pixel unit to another, resulting in gradually reduced brightness from one to another portion of the OLED device. As a result, the OLED display device lacks brightness uniformly. This problem becomes more serious as the size of the OLED display device increases.